Tube mills which are used for processing materials, such as cement raw meal or clinker in grinding chambers are well known.
In some known grinding tube mills, a composite diaphragm is positioned at the outlet end of a grinding chamber. The diaphragm includes a coarse grate which faces the grinding chamber for retaining grinding bodies in the chamber in combination with a sieving screen on the side of the grate downstream from the chamber for sieving the ground product. A mill of this construction is described in U.S. Pat. No. 3,776,477.
A primary purpose of a composite diaphragm is to retain grinding bodies in the grinding chamber thus preventing the grinding bodies from passing from one grinding chamber into the next or out of the mill. A secondary purpose is to protect the sieving screen against the wearing action of the grinding bodies. A third purpose of a composite diaphragm is to maintain a constant sieving area having well defined unclogged sieve openings substantially during the lifetime of the diaphragm to sieve the ground material. In this manner, ground material of specific particle sizes within a specific dimensional range is allowed to pass to the next grinding chamber or out of the mill. Thus the size of the grinding bodies in each grinding chamber may be correlated with the specific particle size range. It should be especially noted that small grinding bodies suitable for grinding a preground material are unable to grind even a small amount of oversized particles.
Many attempts have been made to construct a composite diaphragm fulfilling these purposes in an ideal manner. In the prior art several proposals for composite sieving devices are known comprising two members in close contact with each other wherein the first member faces the inlet of the mill and is a coarse rigid screen which allows all the ground material to pass but prevents grinding bodies from coming into contact with the second member. The second member is a sieving screen having openings small enough to separate off oversize grains. However, in this arrangement, the second member is usually of a slight construction because necessarily the number of openings have to be great in order to obtain sufficient effective area for allowing passage of material and air through the mill. Although the second member is spidery the wear on this member is tolerable because the grinding bodies are kept away from the second member by means of the coarse first member.
However, coarse particles including the oversize grains, worn down grinding bodies, and pieces of broken grinding bodies reach the sieving member through the coarse screen. Disadvantageously, this material and these pieces agglomerate together with fine material to clog the openings so that the total passage and thereby the transport capacity through the diaphragm is reduced. This tendency is increased by the fact that other hard, coarse particles or nibs, become wedged in the mouths of the openings. When the two members are separated, nibs, etc., are trapped and collect between them and may thereby cause an excessive wear of the sieving member or block the passage completely. Further, the coarse screen is usually built up of sections which are exposed to severe wear and have to be changed frequently because the slots in the screen weaken the construction. Further still, the slots are deformed by the hammering action of the grinding bodies. We have invented an improved composite diaphragm which avoids the above-noted limitations of the prior art diaphragms.